Yes, the 550 reads in a vacuum and indicates vacuum in inches of mercury ("hg) It will not however read in microns and the use of a micron gauge is recommended.

Does Testo 549 measure microns?

Answer: Yes, it's the one in the middle; it doesn't have a micron gauge, but you can still use it to vacuum the AC system.

Does the Testo 557 have a micron gauge?

The testo 557 measures vacuum with a barometric pressure compensated relative pressure sensor. The sensor is limited as it has a 500 micron resolution, but it does provide an adequate indication of vacuum for most users.